Constrained CPO

Author: H-G-Hristov

libcxx/include/__ranges/iota_view.h

@@ -375,6 +375,7 @@ namespace views {
 namespace __iota {
 struct __fn {
   template <class _Start>
+    requires(requires(_Start __s) { ranges::iota_view<decay_t<_Start>>(std::forward<_Start>(__s)); })
   _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Start&& __start) const
       noexcept(noexcept(ranges::iota_view(std::forward<_Start>(__start))))
           -> decltype(ranges::iota_view(std::forward<_Start>(__start))) {

libcxx/test/std/ranges/range.factories/range.iota.view/views_iota.pass.cpp

@@ -46,7 +46,14 @@ constexpr void testType(U u) {
 
 struct X {};
 
-constexpr bool test() {
+template <typename IntT>
+concept CanDoubleWrap = requires(IntT i) { std::views::iota(std::views::iota(i)); };
+
+template <typename T>
+concept HasIota = requires(T t) { std::views::iota(t); };
+
+constexpr bool
+test() {
   testType<SomeInt>(SomeInt(10));
   testType<SomeInt>(IntComparableWith(SomeInt(10)));
   testType<signed long>(IntComparableWith<signed long>(10));
@@ -68,6 +75,16 @@ constexpr bool test() {
   {
     static_assert(std::same_as<decltype(std::views::iota), decltype(std::ranges::views::iota)>);
   }
+  { // LWG4096
+    // [[maybe_unused]] auto i1 = std::views::iota(std::views::iota(82));
+    // [[maybe_unused]] auto i2 = std::views::iota(std::views::iota(SomeInt(94)));
+
+    static_assert(!CanDoubleWrap<int>);
+    static_assert(!CanDoubleWrap<SomeInt>);
+    static_assert(!HasIota<decltype(std::views::iota(82))>);
+    static_assert(!HasIota<decltype(std::views::iota(SomeInt(94)))>);
+    static_assert(!std::is_invocable_v<decltype(std::views::iota), decltype(std::views::iota(82))>);
+  }
 
   return true;
 }

libcxx/test/std/ranges/range.factories/range.iota.view/views_iota.verify.cpp

@@ -19,11 +19,11 @@ void test() {
   {
     [[maybe_unused]] auto i1 = std::views::iota(0); // OK
     [[maybe_unused]] auto i2 = std::views::iota(std::views::iota(0));
-    // expected-error-re@*:* {{constraints not satisfied for class template 'iota_view'{{.*}}}}
+    // expected-error@*:* {{no matching function for call to object of type 'const __iota::__fn'}}
   }
   {
     [[maybe_unused]] auto i1 = std::views::iota(SomeInt(0)); // OK
     [[maybe_unused]] auto i2 = std::views::iota(std::views::iota(SomeInt(0)));
-    //expected-error-re@*:* {{constraints not satisfied for class template 'iota_view'{{.*}}}}
+    //expected-error@*:* {{no matching function for call to object of type 'const __iota::__fn'}}
   }
 }